Rick's b.log - entry 2020/04/26

Inside a wireless phone charger

Principles

Many years ago, I had a toothbrush that charged itself automatically and magically when it sat in a special cradle.

It wasn't actually magic, it was something known as induction. It is, in fact, the very basis of how transformers work. An alternating current into one set of windings causes magnetic resonance that induces current in another set of windings. By altering the number of windings one each side, it is possible to fairly easily convert 230V or 110V (mains) down to something more useful like 9V or 6V.
As an aside, a lot of power convertors use something called "switch mode" because it makes them more efficient and a lot smaller. That's why the big lumps that used to plug into the wall to power electric keyboards and such are many times larger than the absolutely tiny things that charge mobile phones and tablets. The big ones just have a regular transformer, bridge rectifier, and regulator (sometimes!) inside. The smaller ones are... much more complicated.

Since we have a method of transferring power from one set of windings to another using resonance, the logical next step was to do it in a way where the windings on each side could actually be in different units. This is how my toothbrush worked. In the centre of the cradle was a little pole that fit into a matching recess in the bottom of the brush. This was intended that you believe that it was a lug that the toothbrush clipped into, which it was, however it was also a piece of iron in there to act as part of the magnetic arrangement.

However mobile phones are tiny and flat, as are the little charging pads, such as the one pictured above. There is no magnet or piece of iron between them, indeed it can even work with a small gap between the two, so how does it work?

The answer is to shift frequencies. Traditional mains transformers work at either 50Hz or 60Hz (depending on where you are in the world). The typical mobile phone charger starts working at 140kHz using two planar (flat) coils. One in the charger, and another in the phone.

The charger sends out pulses at 140kHz (if it is following the widely used Qi standard) and it monitors these pulses (as it forms a resonant system). When a phone is placed on the charging pad, the resonance changes due to the added coil (in the phone) to the charger can detect its presence.
The charger will continue with its generic 140kHz signal for a brief time because data transfer (for setting up the charging parameters) is unidirectional. The phone controls the charger. How it does this is by applying specific loading to the receiving coil. This affects the resonance in a way that can be detected by the charger. By doing this, the phone can request the charger to change frequencies to something more suitable (usually around 105kHz to 205kHz) and also alter the charging current. Typical chargers can output somewhere between 5W and 7.5W (for phones), with coil voltages as high as 50V, but controllable in fine (half volt) steps.

A look inside

Inside is this:

Nothing particularly unusual here, other than the bizarre placement of the LED. There is a coil of fairly thick wire, 10 turns, glued to a piece of iron. This helps inductance and also helps to reduce unwanted emissions. There is a circuit containing four ICs. At a guess, we'll have some sort of controller, a comparator, and two switches. There are no transistors on the board, so I'm guessing one or two of the ICs (likely the two side by side) are taking their function. Given the frequencies and output voltages, probably MOSFETs (basically, a transistor that drives a transistor, to allow it to switch bigger things than a directly controlled transistor would be capable of). As we have resonance at a high frequency, the switching will need to be a pair, a P channel and an N channel (for positive and negative swings).

So, let's get a little closer...

The two devices at the top are a 4953 on the left, which is a dual 30V P-channel MOSFET; and a 9926A on the right, which is a dual N-channel MOSFET. Together these can switch -30V and +20V respectively at currents of 5A (continuous) or up to 20A (pulsed). It looks like they can switch in around twenty nanoseconds (maximum) giving a potential fifty million switches per second, which is 50Mhz, more than sufficient for coping with the 100-200ish kilohertz of the charging station.

To the right is an LM358, this is a dual operational amplifier. Not a comparator. It is likely that this generates the waveform that the MOSFETs switch? I've not looked at it in too much detail.

Finally, the IC with all the legs. This is an 8S003F3, a microcontroller based around an 8 bit STM8 processor core, 8KiB of Flash, 1KiB of RAM, and 128 bytes of EEPROM. There are the usual selection of GPIO, ADC, timers, etc etc. As the Qi protocol is fairly complex in operation, it is not unexpected to find a microcontroller inside.
At a brief look, the 'feel' of the processor is vaguely like an evolved Z80. However, it is worth remembering that the STM8 (itself an evolution of the ST7) is its own design, just as the 8051 and Z80 are different. I use the comparison to give an idea of how it differs from the 6502 and ARM that you'd usually see around this blog.

In operation

Hooking my little oscilloscope to the coil, we can see that it sends a signal of +10V to -7V (rounding to nearest whole). The 'scope claims that it is running at 181kHz, but I'm not sure that we can trust that.

The reason I say we can't trust the frequency is because it is not regular. If we slow the timebase to 50ms, we can see that the pulses actually last for just under 100ms with large gaps in between (notice how the frequency reading is now obviously wrong).

Now if I place my older phone (Samsung S7) in place under the charger (under because everything is upside down due to the clips on the coil), after a moment the waveform changes to a continual ~136kHz with an oscillation of +21V to -9V (or 30V peak to peak). It is interesting that it is unbalanced like this, and not, say, -15V to +15V.

It is also interesting that there are pulses that are stronger than others. I don't know if this is due to inaccuracies in the oscilloscope, the setup, or if it's some subtle kind of continual data transfer from the phone.

At any rate, something interesting to look at on a sunny Sunday morning! And I didn't get oil all over my hands!

Gardening

Just to show that there is more to my life than mowing grass and taking stuff apart, I planted out the sunflowers:

and the tomatoes:

The basil is still too small. Next week, perhaps?

Mom's favourite, the California Poppy (properly named Eschscholzia, which sounds like a reason one would be in hospital), is putting on quite a show:

and a profusion of happy Pansies:

You can see I've put the top of a water bottle around the Amarylis as it is determined to try sending up a second flower and something is determined to call that dinner. Nope, I want a flower this time, thank you...

And with no more outdoor cat, the lizard population has increased. I don't mind the lizards. I just hope the rodent population hasn't increased likewise. I'm not sure I'd trust Wawa to know what to do with a mouse. Certainly, Tiny didn't. The time she met a mouse, they both legged it in opposite directions!

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David Pilling, 28th April 2020, 02:55

So this thing plugs into a USB socket, and thus has 5V at its disposal. The MOSFETS could be a H bridge, connect the coil to 5V one way around and then the other way. The higher voltages might be due to resonance.

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